Yes -- although topology is commonly perceived as a single particle or "classical" phenomenon, it's also valid within the quantum realm and may play a big role in quantum computing.

A group of researchers led by Alexander Szameit, a physics professor at the University of Rostock in Germany, paired the topologically robust properties of light with the interference of photon pairs. And they discovered topology -- an abstract mathematical concept initially developed to classify solid geometries by their global properties -- is also valid within the quantum realm.

Light follows the global characteristics of the waveguide system within topological systems. Even perturbations to the waveguides such as defects, vacancies, and disorder won't divert its path.

This work, which was a collaboration with colleagues from the Albert Ludwig University of Freiburg, opens numerous intriguing questions involving topological robustness of entanglement and even nonlocal topological effects. It's a big deal because topology may soon play a key role in the transmission and processing of quantum information.

Szameit's group explores integrated photonic waveguide circuits and quantum light. "Originally, we used classical light for quantum simulations of solid-state phenomena," he says. "But we realized there's much to be discovered by extending our research to the quantum features of light, such as entanglement and nonlocality. This is when it started, out of curiosity, 12 years ago. Everyone should remember the name of Max Ehrhardt, my Ph.D. student who did the work, because he has a truly ingenious mind."

The quantum effect the team's experiment is based on is Hong-Ou-Mandel interference. In 1987, the three physicists observed the behavior of photon pairs within a beam splitter during an experiment and found that a photon, which interferes with itself due to its behavior as an electromagnetic wave, can also form interference patterns with other light particles. Beyond entanglement as a fundamental feature of quantum light particles, this groundbreaking discovery is an ingredient for optical quantum technologies, including quantum computers.

"Two indistinguishable photons enter a network and bunch up," explains Szameit. "When they enter a beam splitter from different sides they will always exit it together -- although we can't tell which side of the beam splitter the pair will exit. This effect is rather fragile: the splitter requires a perfect 50/50 splitting ratio, and any deviation immediately results in a degradation of the bunching."

Topological photonics is another topic Szameit and colleagues have focused on for 10 years. "By using topological concepts, we can generate light waves that are robust to distortions, which means they can travel even around corners and defects," he says. "Our idea was to combine both effects to get topologically protected Hong-Ou-Mandel interference."